Micrologix 1500 coms monitoring


Thread Starter


I have a PLC that I think is receiving a rogue message. There is a bit that is set for no apparent reason and there is nothing in the local ladder logic to set that bit, so I suspect it is being set from another PLC on the network. I've scoured all the other sites but cannot find anything that might be causing the problem. I'm wondering if there is a way to capture the most recent packet received on channel 1 and store that. This way when the bit in question is set I can hopefully see what node the last packet was received from.

I'm using the DF1 Radio modem protocol. If anyone has any other ideas of how I can track this down I'd be grateful for the suggestions.


Jerry Miille

Since you are using the DF-1 Radio Modem Protocol, you must be connected to the serial port using RS232 connections.

I recommend Serial Port Async from Frontline Test Equipment. They offer a free 30 day trial download of their software. The only thing you will have to do is kludge up a "custom cable" that will bring the TXD and RXD lines to the RXD inputs of two serial ports on your PC.

The software will take it from there. You can record messages to disk and then search the recorded messages for the rogue message.

I am not affiliated with this company at all but use this product daily. If you deal with serial communication then you should have this program. It is not expensive.

Jerry Miille
Miille Applied Research Co., Inc. http://www.miille.com/

Bob Peterson

Some of the SLC products allow you to protect a data file from being changed in this way. Not sure about a 1500.